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Unanswered Questions

Electromagnetics: Transformers, Generators, Motors and Other AC Machinery. September 2017

I’m trying to make sense of everything coiled, but the only thing getting wound up is me. I thought I knew a bit about electromagnetics, but recently I’ve been trying to make sense of all of these fields and flows.

What is the difference between the magnetic field and the flux? How does flux work in a transformer or a generator? Does anyone really understand Maxwell’s Equations?

So many textbooks dealing with electromagnetism speak in equations instead of English. I want to know HOW it all works, not just how to compute these things. Am I just reading the wrong books. Can you help me figure out what the flux is going on?

Taylor Street
Felton, CA


The magnetic flux is the magnetic field. Flux usually refers to the portion of a magnetic field that passes through a particular imaginary surface, usually bounded by a circuit.

I have discovered that there is almost universal confusion and misunderstanding concerning electromagnetic induction. For example, some of the answers to your question that were published in N&V contain misinformation. Walter Lewin’s lectures were recommended. I watched one and found that he made some egregious conceptual errors: He maintained that one could measure two different voltages between the same two points of a circuit! He also conflates Faraday’s Law with one of Maxwell’s equations. Another answer mentioned moving magnetic field lines cutting through a wire and generating electricity. This does not happen. In addition, it seems that magnetic fields do not move. Look up Faraday’s Paradox in Wikipedia and Bruce DePalma’s N-Machine in YouTube.

Here is a quick lesson on induction. There are two distinct principles that are called induction. One is motional emf. It is just a direct application of the definition of the magnetic field. If a conductor moves through a magnetic field such that it cuts the magnetic field lines, a non-electrostatic emf is generated in the conductor. This is how generators work.

The second principle is that a time-varying magnetic field is accompanied by an electric field. This is formulated by one of Maxwell’s equations, sometimes called the Maxwell-Faraday Law. This is how alternators work. It is not the moving magnetic field lines produced by the primary winding of a transformer that induce a current in the secondary winding. It is the electric field produced by the changing magnetic field.

Faraday’s Law causes a lot of confusion because it seems to include both principles of induction, although it does not. In most circumstances it allows one to calculate the emf of a circuit using the second principle when, in actuality, the first principle is operative. The only difference in the equations of Faraday’s Law and the Maxwell-Faraday Law is that Faraday’s Law uses the ordinary derivative and the Maxwell-Faraday Law uses the partial derivative. The partial derivative eliminates the effect of any motion of the circuit. Faraday’s Law only applies to a circuit. The Maxwell-Faraday Law does not require a circuit or even a conductor.

Michael S. La Moreaux
Ann Arbor, MI

The magnetic field is a force field that exists around any current carrying conductor. When the wire is wound as a solenoid, the fields add and it acts like a magnet if the current is DC. If the current is AC, the north and south poles continuously reverse.

The force field is continuous; there are no lines but it is convenient to measure the density of the field in Webers per square meter or Gauss, depending on the system. The density of the field is sometimes called flux.

Russell Kincaid
Milford, NH

Magnetics can be really fascinating. I could sit here and type, but if you google left hand motor rule, it has nice diagrams to explain the functions. Motors are different so you need the right hand rule.

I was very confused as you are, but a instructor in the marines demonstrated the hand rules and it then made sense. Good luck and enjoy the hand rules.

Thomas Sides
Phoenix, AZ

My best attempt to explain a transformer: Any time you move a magnet near a wire, you generate electricity in the wire. Especially so if the magnetic field lines “cut through” the wire as they are moving.

The secondary (output) windings on the transformer are constantly being “cut” by the moving magnetic field from the primary, so they generate electricity. The job of the primary (input) windings is to set up the moving magnetic field in the core and in the secondary. The primary is acting as an electromagnet.

The “movement” results from the fact that the current in the primary is AC and constantly changing. A transformer will work on AC but not on DC. That’s it, fools rush in where angels fear to tread.

Chip Veres
Miami, FL

I am also struggling with the same thing. I recently found a series of great Electricity and Magnetism lectures that are easy on the math and have lots of practical demonstrations on youtube. These are by Walter Lewin at MIT and you can find them on youtube by searching for 8.02X.

The first lecture is at . Also, there are lots of other free video lectures available for free from MIT open courseware as well at

Keith Ujvary
Gibsons, BC

Sharing A Monitor With Two PCs January 2018

I have two desktop computers and three 32" HP Pavillion 32Q monitors being used by two people at the same time. One of the monitors is assigned to each computer and depending on the applications being used, the third one goes to whomever benefits most by having two screens. I have to physically disconnect and reconnect the third screen when it needs to be moved to the other PC.

How can I do this without having to crawl uder the table to make the switch? I've looked at KVM switches, but I only need to share the third monitor, not the keyboard, mouse, etc. The monitors use the display port input and run at 2560 x 1440. I would just add another monitor to run two with each PC, but there isn't enough room in the work area.

Vicente Benedetti
Athens, TX

Low-Cost 3D Printers January 2018

I’ve been eyeing 3D printers for a while, and there are now basic units in the $100 range.

However, it appears that software can cost between free and several hundred dollars depending on features and capabilities. I like free. Is anyone using free software to create and print using one of these low-cost printers? If so, which software and printer are you using that you would recommend? Any caveats?

Brenda Spellman
Green Bay, WI

Chip Replacement January 2018

I have an old guitar “echo” pedal that is dead. After some troubleshooting, it appears the SAD1024 chip in the unit has given up the ghost and I have been unable to find a replacement chip.

Is there a substitute for it or a source for replacing it?

Mike Styles
Pennsauken, NJ

Converting S-Video to Composite Video January 2018

I have a newer HD LED television that does NOT have an S-Video input jack! My old VCR and tapes are all S-VHS, so I need a schematic for building a converter for S-Video to Composite if I want to watch my old family movies. Please help!

Henry Vaden
Whitestone, NY

Solid-State Tube Replacement December 2017

I’m refurbishing a tube-type Hallicrafters shortwave receiver. I was planning on using a solid-state plug-in replacement for the rectifier tube; mostly because I can’t find the tube.

I’ve been told that a solid-state rectifier could result in higher voltage, and may blow the filter capacitors and run the tubes at a non-linear part of their operational curves. Can someone confirm or explain if this is a good idea or not?

Matthew Stiefel
Steelville, MO

VGA To LCD December 2017

I would like to be able to drive a 4x20 line LCD with a VGA output from an old computer. Is there a simple interface to do this?

Evan Lee
Elizabethtown, IL

AMP Clamp December 2017

Could someone explain in simple terms how an AMP clamp works? Does it have a transformer in it or Hall-effect sensors or similar?

Kevin Champion
Cleveland, OH

VAX VMS Emulation December 2017

Is there an emulator for a Digital Equipment Corporation’s VAX VMS 4.7 machine, either running on an SBC (Single Board Computer), or possibly an image that can run as a virtual machine in VMWare or something similar?

I want to create a four-node VAX cluster like the one I used to work on, and would love to see it sitting on my desk as a stack of Raspberry Pi boards.

Troy Thoele
Huntsville, TX

Troubleshooting Amp Problems November 2017

I picked up a CrownDC150a power amp at a garage sale. When I tested it on my bench, one side is rather distorted but then clears up once the amp warms up, making it hard to trace. Is this a common problem with these amps? Any troubleshooting tips are appreciated.

Rich Wortman
Kankakee, IL

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